Vienna is the world leader in espionage with at least 7,000 spies plying their
trade in the Austrian capital

Its reputation as a centre of espionage long predates its notoriety as the setting for the 1949 film The Third Man but only now can a figure be put on the number of spies operating in Vienna.

A survey compiled by experts in spying activities in the Austrian capital shows that at least 7,000 agents work undercover in the city.

As neutral country, Vienna was a Cold War spying hub where both sides were able to ply their trade and openly dealt with each other. Its allure was explained in the opening sequence in the Third Man when the narrator observed that Vienna allows agents a free run: “We’d run anything if people wanted it enough, and had the money to pay.”

Unrivalled as a Cold War playground, the proximity of the Balkans following the collapse of the Soviet bloc, proved to be a fresh start for the second oldest profession.

Since the 2001 attacks on America, the city has also been a significant theatre for competing Middle East interests.

According to Emil Bobi, the author of Die Schattenstadt (The Void), a history of spying in Vienna, state-sponsored subterfuge stretches back to the Austro-Hungarian empire.

The country’s laws have never been updated despite the collapse of the empire, two world wars and the East-West stand-off. As a result the only spying activities that are illegal in Austria are direct targeting of Austrian state secrets.

“Every second diplomat in Vienna can be linked to his or her country’s intelligence agencies,” said Mr Bobi. “Every embassy is overcrowded with personnel.”

Gert René Polli, the former chief of BVT, the country’s counter-terrorism agency, said the 7,000 figure was a base level that could be supplemented by others linked to the trade.

“Vienna is a stock exchange of information. We have the most liberal laws governing this activity in the world,” he told the Telegraph. “It is also a nice place for spies to live and bring their families with good education and health services after difficult postings in Serbia, Iraq or Afghanistan.”

As the host country for a raft of United Nations and international organisation, Vienna has a large number of diplomatic delegations that provide a perfect front for spying. Not only is it the home of the International Atomic Energy Agency (IAEA), which deals with nuclear proliferation, but it also hosts the Organisation for Security and Cooperation in Europe (OSCE) and the Organisation of Petroleum Exporting Countries (Opec).

While the overlapping spheres of spies shift in importance with international events, the traditional rivalry between the US and Russia remains the strongest single practice with both maintaining the largest numbers of spies.

This was tacitly acknowledged when the city was the venue chosen for the biggest “spy swap” of recent years. In broad daylight 14 American and Russian spies, including Anna Chapman, were exchanged on the runway of Vienna’s airport in 2010.

Espionage can hide in plain view because there are almost no calls for change.

“It’s life here. It’s part of Vienna’s culture,” said Mr Bobi. “Viennese society is built on secrets and people live to have secrets. If you have a secret in Vienna you are somebody.”